Wavelength division multiplexed (WDM) optical communication systems are known to include one or more photonic integrated circuits (PICs) in which multiple optical signals, each having a different wavelength or associated carrier frequency, are combined into a modulated output signal for transmission over an optical fiber. Such systems typically include transmitters having a laser supplying light at a carrier frequency, a modulator configured to modulate the light output of the laser, and an optical combiner to combine each of the modulated outputs into the modulated output signal. The carrier frequencies associated with each of the modulated outputs, collectively the frequencies defining a carrier frequency grid, or simply frequency grid, may be spectrally spaced from each other to define a channel spacing with respect to the grid. The WDM optical communication system may include multiple PIC devices, the outputs of each being spectrally combined or interleaved to form a combined output signal for transmission over an optical fiber. The frequency grids of each of such PIC devices may be spectrally spaced or offset from each other such that a portion of each frequency grid overlaps a portion of the remaining frequency grids. Thus, each adjacent signal channel may be from different wavelength grids, the adjacent signal channels defining a channel spacing with respect to the interleaved signal.
Historically, such modulated output signals were amplitude or intensity modulated. More recently, however, more advanced transmission systems, such as coherent systems, use more complex modulation formats. Such complex modulation formats may employ phase-shift keying (PSK), for example, which offer higher capacity than the intensity modulated signals. Some examples of modulation formats which incorporate PSK include binary phase shift keying (“BPSK”), quadrature phase shift keying (“QPSK”), differential phase-shift keying (“DPSK”), and polarization multiplexed differential phase-shift keying (PM-DPSK), to name a few.
As with other transmission systems, coherent transmission systems may provide for a data capacity over a corresponding link of a given distance or reach, the capacity provided within a margin, such as a bit error rate for example. However, links employed for transmission of optical signals may include impairments which may limit the performance of the transmission system. Such impairments may include, for example, various forms of dispersion due to random imperfections and asymmetries of the optical fiber associated with the link, such as polarization mode dispersion or cross phase modulation chromatic dispersion. For purposes herein, impairments shall include any linear or non-linear impairment which may impact the integrity of a transmitted optical signal. Such impairments may be associated with the optical signal itself, such as optical power or a modulation scheme employed to modulate the optical signal, or may be associated with structure of the transmission system, such as an optical fiber over which the optical signal is transmitted.
What is needed is a system which monitors characteristics of optical signals, as part of a wavelength division multiplexed signal for example, in order to optimize the optical signals in light of impairments which may be present.